TrueType

TrueType has become the most common format for fonts on both the Mac OS and Microsoft Windows operating systems.
TrueType is an outline font standard originally developed by Apple Computer in the late 1980s. It was later adopted by Microsoft.

OpenType

OpenType is a scalable computer font format. It was developed by Microsoft Corporation with the contribution of Adobe Systems. The format is based on its predecessor TrueType, retaining TrueType's basic structure and adding many intricate data structures for prescribing typographic behavior. OpenType fonts are used commonly today on the major computer platforms.

Type 1

Type 1 (also known as PostScript Type 1) fonts are based on Adobe's PostScript language and are widely used in desktop publishing. Type 1 fonts consist usually of two files — one containing the outline font data (extension: .pfb) and another containing the font metrics (extension: .pfm). On the Mac OS the outline font data is stored in a suitcase along with the associated bitmap fonts for on-screen display.